Hard to crack is usually good, means the egg has a nice strong shell. Black inside sounds like some sort of bacteria, but did your customer complain of a bad smell? Rotten eggs smell bad. Sometimes due to glitches in egg production, there will be blood in the egg, but it looks red, not black. There are other abnormalities of eggs, but I've never heard of anything being black that wasn't rot.
I have found eggs with little hairline cracks in them that are almost impossible to see. I think bacteria could enter through a crack and cause the problem you had. You can candle eggs with a flashlight to look for cracks. The commercial egg producers have machines that check every egg.
I was reading some more, and it is rare, but possible, that the hen has a bacterial infection in the oviduct that infected the yolk before it was encased in the shell. This would make sense considering your egg was only a week old. Not sure how to tell which hen it is or how to treat.
I also read that there are some black yolks that don't smell and that cotton seed meal can make yolks dark.
